The present invention relates to an oxygen sensing element of the type having an oxygen ion conductive solid electrolyte layer supported on a ceramic substrate and a pair of electrode layers formed adjacent the solid electrolyte layer.
Oxygen sensors utilizing an oxygen ion conductive solid electrolyte such as zirconia stabilized with calcia or yttria are widely used for detection of concentrations of oxygen in various gas atmospheres. Particularly in the automotive industry it has become common to install such an oxygen sensor in the exhaust system as a means for detecting actual air/fuel ratio values in the engine. In this the automotive field, a recent trend is to miniaturize the oxygen sensitive element of the sensor by constructing it as a laminiate of thin, film-like layers on a ceramic substrate of very small size. The principal part of the laminate is a combination of solid electrolyte layer and a pair of electrode layers, which constitutes a sort of oxygen concentration cell. Usually an electric heater is embedded in the substrate to maintain the oxygen sensing element in operation at a sufficiently elevated temperature such as 600.degree.-800.degree. C. because the solid electrolyte oxygen concentration cell does not properly function at temperatures below a certain level such as about 400.degree. C.
In many cases the two electrode layers in an oxygen sensing element of the laminate type are formed on the two opposite sides of the solid electrolyte layer so that one of the electrode layers is closely sandwiched between the ceramic substrate and the solid electrolyte layer. However, it is also possible to construct an oxygen sensing element that operates on the same principle by forming both of the two electrode layers on the same side of the solid electrolyte layer with a narrow gap between the two electrode layers. In this case, the solid electrolyte layer makes direct contact with the surface of the ceramic substrate at least in a portion existing between the two electrode layers.
In producing a solid electrolyte oxygen sensing element the content of the stabilizing oxide such as yttria in the solid electrolyte such as zirconia is carefully controlled to thereby control the proportion of cubic zirconia to monoclinic zirconia in the resultant solid electrolyte and to afford the solid electrolyte with optimal electric characteristics. However, when the solid electrolyte layer in the oxygen sensing element is in direct contact with the substrate of a ceramic material such as alumina, there occurs a considerable change in the content of the stabilizing oxide in the solid electrolyte layer during sintering of the solid electrolyte in the production of the oxygen sensing element and also during long use of the oxygen sensing element in a heated state. The reason is presumed to be diffusion of the stabilizing oxide from the solid electrolyte layer into the ceramic substrate. Such a change in the content of the stabilizing oxide in the solid electrolyte layer is accompanied by changes is the resistance and polarizing characteristic of the solid electrolyte layer and, hence, results in a considerable change in the output characteristic of the oxygen sensing element.